The CD4+ lymphocyte count is a crucial parameter for the assessment of the integrity of the immune system of patients infected with the human immunodeficiency virus (HIV). T-lymphocyte counts are not static, however, and may be affected by a number of variables, including circadian rhythm,1 smoking,2 pregnancy,3 and immunization.4 In addition, a number of limited studies have suggested that infections and specific illnesses may also affect the CD4+ cell count1- 4; thus, immune function evaluation of hospitalized HIV-positive patients is often deferred until recovery. There are no data addressing more broadly the effect of generalized severe illness on the lymphocyte profiles of non–HIV-infected patients. This prospective observational study was designed to evaluate lymphocyte subsets in an HIV-negative population that required intensive care unit admission during acute illness.